Extrinsic and intrinsic factors affecting the metamorphic rate of Tripneustes gratilla (Echinodermata: Echinoidea)

Abstract

Experiments were conducted to determine the effects of water conditioned with Sargassum (i.e. fresh and ground Sargassum and excreta of Sargassum-fed Tripneustes gratilla) as well as the effects of adult sea urchins (i.e. Salmacis sphaeroides and T. gratilla), larval age and parentage on the settlement and metamorphic rate of full-sibling batches of T. gratilla larvae. The majority of larvae (35 to 57 d old) settled within the first hour of the assays, but significantly more settled and metamorphosed completely in the conditioned water treatments than in the control (boiled filtered seawater). Completely metamorphosed larvae in the control treatment indicate that specific cues are not necessary to induce settlement and metamorphosis of competent larvae. However, habitat cues enhanced metamorphic rate significantly. Complete metamorphosis occurred earlier in Sargassum treatments, and the number of completely metamorphosed larvae was significantly higher in the ground Sargassum treatment. Observations of early juvenile aggregations on Sargassum further suggest that algal metabolites may enhance benthic recruitment. The sensitivity of sea urchin larvae to waterborne cues differed among larval age groups. Older batches of larvae metamorphosed faster, particularly in the ground Sargassum treatment, while the intermediate age group also had high metamorphic rates in both sea urchin treatments. There were significant differences in metamorphic rate among full sibling batches of larvae of the same age, as well as significant interaction between batch and treatment over time. Results indicate that differences in the age of competent larvae and genetic factors affect the ability of larvae to detect cues to initiate and complete metamorphosis. © Inter-Research 2010, www.int-res.com.